MIDI Messages

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MIDI Messages

A MIDI message is made up of an eight-bit status byte which is generally
followed by one or two data bytes. There are a number of different types of MIDI
messages. At the highest level, MIDI messages are classified as being either
Channel Messages or System Messages.

A D V E R T I S E M E N T

Channel messages are those which apply to a
specific Channel, and the Channel number is included in the status byte for
these messages. System messages are not Channel specific, and no Channel number
is indicated in their status bytes.

Channel Messages may be further classified as being either Channel Voice
Messages, or Mode Messages. Channel Voice Messages carry musical performance
data, and these messages comprise most of the traffic in a typical MIDI data
stream. Channel Mode messages affect the way a receiving instrument will respond
to the Channel Voice messages.

Channel Voice Messages

Channel Voice Messages are used to send musical performance information. The
messages in this category are the Note On, Note Off, Polyphonic Key Pressure,
Channel Pressure, Pitch Bend Change, Program Change, and the Control Change
messages.

Note On / Note Off / Velocity

In MIDI systems, the activation of a particular note and the release of the
same note are considered as two separate events. When a key is pressed on a MIDI
keyboard instrument or MIDI keyboard controller, the keyboard sends a Note On
message on the MIDI OUT port. The keyboard may be set to transmit on any one of
the sixteen logical MIDI channels, and the status byte for the Note On message
will indicate the selected Channel number. The Note On status byte is followed
by two data bytes, which specify key number (indicating which key was pressed)
and velocity (how hard the key was pressed).

The key number is used in the receiving synthesizer to select which note
should be played, and the velocity is normally used to control the amplitude of
the note. When the key is released, the keyboard instrument or controller will
send a Note Off message. The Note Off message also includes data bytes for the
key number and for the velocity with which the key was released. The Note Off
velocity information is normally ignored.

Aftertouch

Some MIDI keyboard instruments have the ability to sense the amount of
pressure which is being applied to the keys while they are depressed. This
pressure information, commonly called "aftertouch", may be used to control some
aspects of the sound produced by the synthesizer (vibrato, for example). If the
keyboard has a pressure sensor for each key, then the resulting "polyphonic
aftertouch" information would be sent in the form of Polyphonic Key Pressure
messages. These messages include separate data bytes for key number and pressure
amount. It is currently more common for keyboard instruments to sense only a
single pressure level for the entire keyboard. This "Channel aftertouch"
information is sent using the Channel Pressure message, which needs only one
data byte to specify the pressure value.

Pitch Bend

The Pitch Bend Change message is normally sent from a keyboard instrument in
response to changes in position of the pitch bend wheel. The pitch bend
information is used to modify the pitch of sounds being played on a given
Channel. The Pitch Bend message includes two data bytes to specify the pitch
bend value. Two bytes are required to allow fine enough resolution to make pitch
changes resulting from movement of the pitch bend wheel seem to occur in a
continuous manner rather than in steps.

Program Change

The Program Change message is used to specify the type of instrument which
should be used to play sounds on a given Channel. This message needs only one
data byte which specifies the new program number.

Control Change

MIDI Control Change messages are used to control a wide variety of functions
in a synthesizer. Control Change messages, like other MIDI Channel messages,
should only affect the Channel number indicated in the status byte. The Control
Change status byte is followed by one data byte indicating the "controller
number", and a second byte which specifies the "control value". The controller
number identifies which function of the synthesizer is to be controlled by the
message. A complete list of assigned controllers is found in the MIDI 1.0
Detailed Specification.

- Bank Select

Controller number zero (with 32 as the LSB) is defined as the bank select.
The bank select function is used in some synthesizers in conjunction with the
MIDI Program Change message to expand the number of different instrument sounds
which may be specified (the Program Change message alone allows selection of one
of 128 possible program numbers). The additional sounds are selected by
preceding the Program Change message with a Control Change message which
specifies a new value for Controller zero and Controller 32, allowing 16,384
banks of 128 sound each.

Since the MIDI specification does not describe the manner in which a
synthesizer's banks are to be mapped to Bank Select messages, there is no
standard way for a Bank Select message to select a specific synthesizer bank.
Some manufacturers, such as Roland (with "GS") and Yamaha (with "XG") , have
adopted their own practices to assure some standardization within their own
product lines.

- RPN / NRPN

Controller number 6 (Data Entry), in conjunction with Controller numbers 96
(Data Increment), 97 (Data Decrement), 98 (Registered Parameter Number LSB), 99
(Registered Parameter Number MSB), 100 (Non-Registered Parameter Number LSB),
and 101 (Non-Registered Parameter Number MSB), extend the number of controllers
available via MIDI. Parameter data is transferred by first selecting the
parameter number to be edited using controllers 98 and 99 or 100 and 101, and
then adjusting the data value for that parameter using controller number 6, 96,
or 97.

RPN and NRPN are typically used to send parameter data to a synthesizer in
order to edit sound patches or other data. Registered parameters are those which
have been assigned some particular function by the MIDI Manufacturers
Association (MMA) and the Japan MIDI Standards Committee (JMSC). For example,
there are Registered Parameter numbers assigned to control pitch bend
sensitivity and master tuning for a synthesizer. Non-Registered parameters have
not been assigned specific functions, and may be used for different functions by
different manufacturers. Here again, Roland and Yamaha, among others, have
adopted their own practices to assure some standardization.

Channel Mode Messages

Channel Mode messages (MIDI controller numbers 121 through 127) affect the
way a synthesizer responds to MIDI data. Controller number 121 is used to reset
all controllers. Controller number 122 is used to enable or disable Local
Control (In a MIDI synthesizer which has it's own keyboard, the functions of the
keyboard controller and the synthesizer can be isolated by turning Local Control
off). Controller numbers 124 through 127 are used to select between Omni Mode On
or Off, and to select between the Mono Mode or Poly Mode of operation.

When Omni mode is On, the synthesizer will respond to incoming MIDI data on
all channels. When Omni mode is Off, the synthesizer will only respond to MIDI
messages on one Channel. When Poly mode is selected, incoming Note On messages
are played polyphonically. This means that when multiple Note On messages are
received, each note is assigned its own voice (subject to the number of voices
available in the synthesizer). The result is that multiple notes are played at
the same time. When Mono mode is selected, a single voice is assigned per MIDI
Channel. This means that only one note can be played on a given Channel at a
given time. Most modern MIDI synthesizers will default to Omni On/Poly mode of
operation. In this mode, the synthesizer will play note messages received on any
MIDI Channel, and notes received on each Channel are played polyphonically. In
the Omni Off/Poly mode of operation, the synthesizer will receive on a single
Channel and play the notes received on this Channel polyphonically. This mode
could be useful when several synthesizers are daisy-chained using MIDI THRU. In
this case each synthesizer in the chain can be set to play one part (the MIDI
data on one Channel), and ignore the information related to the other parts.

Note that a MIDI instrument has one MIDI Channel which is designated as its
"Basic Channel". The Basic Channel assignment may be hard-wired, or it may be
selectable. Mode messages can only be received by an instrument on the Basic
Channel.

System Messages

MIDI System Messages are classified as being System Common Messages, System
Real Time Messages, or System Exclusive Messages. System Common messages are
intended for all receivers in the system. System Real Time messages are used for
synchronization between clock-based MIDI components. System Exclusive messages
include a Manufacturer's Identification (ID) code, and are used to transfer any
number of data bytes in a format specified by the referenced manufacturer.

System Common Messages

The System Common Messages which are currently defined include MTC Quarter
Frame, Song Select, Song Position Pointer, Tune Request, and End Of Exclusive
(EOX). The MTC Quarter Frame message is part of the MIDI Time Code information
used for synchronization of MIDI equipment and other equipment, such as audio or
video tape machines.

The Song Select message is used with MIDI equipment, such as sequencers or
drum machines, which can store and recall a number of different songs. The Song
Position Pointer is used to set a sequencer to start playback of a song at some
point other than at the beginning. The Song Position Pointer value is related to
the number of MIDI clocks which would have elapsed between the beginning of the
song and the desired point in the song. This message can only be used with
equipment which recognizes MIDI System Real Time Messages (MIDI Sync).

The Tune Request message is generally used to request an analog synthesizer
to retune its' internal oscillators. This message is generally not needed with
digital synthesizers.

The EOX message is used to flag the end of a System Exclusive message, which
can include a variable number of data bytes.

System Real Time Messages

The MIDI System Real Time messages are used to synchronize all of the MIDI
clock-based equipment within a system, such as sequencers and drum machines.
Most of the System Real Time messages are normally ignored by keyboard
instruments and synthesizers. To help ensure accurate timing, System Real Time
messages are given priority over other messages, and these single-byte messages
may occur anywhere in the data stream (a Real Time message may appear between
the status byte and data byte of some other MIDI message).

The System Real Time messages are the Timing Clock, Start, Continue, Stop,
Active Sensing, and the System Reset message. The Timing Clock message is the
master clock which sets the tempo for playback of a sequence. The Timing Clock
message is sent 24 times per quarter note. The Start, Continue, and Stop
messages are used to control playback of the sequence.

The Active Sensing signal is used to help eliminate "stuck notes" which may
occur if a MIDI cable is disconnected during playback of a MIDI sequence.
Without Active Sensing, if a cable is disconnected during playback, then some
notes may be left playing indefinitely because they have been activated by a
Note On message, but the corresponding Note Off message will never be received.

The System Reset message, as the name implies, is used to reset and
initialize any equipment which receives the message. This message is generally
not sent automatically by transmitting devices, and must be initiated manually
by a user.

System Exclusive Messages

System Exclusive messages may be used to send data such as patch parameters
or sample data between MIDI devices. Manufacturers of MIDI equipment may define
their own formats for System Exclusive data. Manufacturers are granted unique
identification (ID) numbers by the MMA or the JMSC, and the manufacturer ID
number is included as part of the System Exclusive message. The manufacturers ID
is followed by any number of data bytes, and the data transmission is terminated
with the EOX message. Manufacturers are required to publish the details of their
System Exclusive data formats, and other manufacturers may freely utilize these
formats, provided that they do not alter or utilize the format in a way which
conflicts with the original manufacturers specifications.

Certain System Exclusive ID numbers are reserved for special protocols. Among
these are the MIDI Sample Dump Standard, which is a System Exclusive data format
defined in the MIDI specification for the transmission of sample data between
MIDI devices, as well as MIDI Show Control and MIDI Machine Control.

Running Status

Since MIDI data is transmitted serially, it is possible that musical events
which originally occurred at the same time and must be sent one at a time in the
MIDI data stream may not actually be played at exactly the same time. With a
data transmission rate of 31.25 Kbit/s and 10 bits transmitted per byte of MIDI
data, a 3-byte Note On or Note Off message takes about 1 ms to be sent, which is
generally short enough that the events are perceived as having occurred
simultaneously. In fact, for a person playing a MIDI instrument keyboard, the
time skew between playback of notes when 10 keys are pressed simultaneously
should not exceed 10 ms, and this would not be perceptible.

However, MIDI data being sent from a sequencer can include a number of
different parts. On a given beat, there may be a large number of musical events
which should occur simultaneously, and the delays introduced by serialization of
this information might be noticeable. To help reduce the amount of data
transmitted in the MIDI data stream, a technique called "running status" may be
employed.

Running status considers the fact that it is very common for a string of
consecutive messages to be of the same message type. For instance, when a chord
is played on a keyboard, 10 successive Note On messages may be generated,
followed by 10 Note Off messages. When running status is used, a status byte is
sent for a message only when the message is not of the same type as the last
message sent on the same Channel. The status byte for subsequent messages of the
same type may be omitted (only the data bytes are sent for these subsequent
messages).

The effectiveness of running status can be enhanced by sending Note On
messages with a velocity of zero in place of Note Off messages. In this case,
long strings of Note On messages will often occur. Changes in some of the MIDI
controllers or movement of the pitch bend wheel on a musical instrument can
produce a staggering number of MIDI Channel voice messages, and running status
can also help a great deal in these instances.